Originally posted by Maricarl@Jul 6 2005, 09:45 PMWhy is varnish applied in transformer?
What is the effect if transformer is not applied?
What is the composition of varnish?

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I think the "varnish" you're referring to is the insulation on the windings and the iron core. The purpose of insulating the windings is to keep the different phases/poles/coils insulated from each other, and to keep the coils insulated from the iron core. If there were no insulation, the windings would short out against each other, or ground out against the core.

Within the iron core electrons are manipulated around by induction coming from the energized coils and this, in turn, builds a little extra heat, which amounts to a power loss. The phenomenon is known as eddy (stray) currents. Laminating the segments of the iron core reduces this power loss, and insulating the iron core segments from each other furthers the protection against this loss.

I have no idea what the exact (or even general) composition of the insulating varnish is, but there are several classes of insulation. They are usually given alphabetic names (Class F, etc) which refer directly the temperature rating of that class.